Prior art fixed pitch propellers can be designed to operate most efficiently either at climb speeds of the aircraft or below. In the alternative a conventional fixed pitch propeller can be designed to operate most efficiently at higher cruise speeds of the aircraft upon which they are installed. The purpose of the present invention is to provide an improved fixed pitch propeller which will operate efficiently in both regimes, and which will also exhibit improved performance of the aircraft upon which it is installed both at take off and at speeds in excess of cruise speed.
In aircraft equipped with conventional fixed pitch propellers the speed of rotation of the propeller is related to the throttle setting of the engine driving the propeller, and to the airspeed of the aircraft. A given propeller geometry will be most efficient at only one aircraft speed and at a particular engine speed. Variable pitch propellers that maintain a preset engine speed do overcome, and/or alleviate the inherent "single speed" design of conventional fixed pitch propellers. However, both fixed pitch propellers and variable pitch propellers are built on the premise that the relationship of blade pitch angle at a particular radial station of the blade is dictated primarily by the aircraft's forward speed, and engine speed, hence the station's blade rotational speed.
More specifically, fixed pitch propellers have traditionally been made with blade angles that are related to radial stations along the blade such that the trigonometric tangent of the blade angle (.beta.) at a particular radial station is inversely proportional to the radial distance (R) of the station from the blade's rotational axis (tan.beta.=.sup.k.sub.R). In a "cruise" prop this constant (k) is greater that it would be in a "climb" prop.
If we look at the helical path that the rotating propeller blade tip describes in space, for example, the "pitch distance" of the helix is a function of the propeller's speed, or more correctly velocity, and this velocity has a direction that is dictated by the rotational speed of the propeller and by the forward speed of the aircraft. The propeller is a rotating wing that generates lift (thrust) as it moves through the air. According to aerodynamic theory any wing has an optimum angle of attack that provides the highest ratio of lift (or thrust) to drag. Therefore, the propeller can only operate at optimum efficiency at a particular speed (corresponding to a particular forward speed and rotational speed). In a conventional fixed pitch propeller for example, the "pitch distance" of the ideal helix might be 72". This "pitch distance" is dictated by the blade angle at the tip, and this ideal helix also dictates blade angles at the various blade stations as described above. That is, tangent .beta.=.sup.k.sub.R.